PROJECT SUMMARY/ABSTRACT Allogeneic bone marrow transplantation (allo-BMT) can cure blood cancers and other disorders, but is limited by life-threatening graft-versus-host-disease (GVHD). After myeloablative conditioning and allo-BMT, donor- derived T cells encounter foreign tissue antigens and other critical signals that induce pathogenic alloimmunity and GVHD. To date, our understanding of these signals remains incomplete. We discovered that juxtacrine Notch signals induced by Delta-like1/4 (Dll1/4) ligands are key drivers of lethal GVHD. Alloreactive T cells received Notch signals from non-hematopoietic fibroblastic stromal cells in the spleen and lymph nodes, but not from hematopoietic antigen-presenting cells. Dll1/4 inactivation in Ccl19-Cre+ fibroblastic stromal cells protected recipients from lethal GVHD in a mouse model of myeloablative allo-BMT. However, it is unknown whether fibroblastic stromal cells are essential for GVHD pathogenesis only after myeloablative conditioning and if they can function as a cellular source of alloantigens in addition to presenting Notch ligands. The overall goal of this proposal is to study the role and regulation of fibroblastic stromal cells in T cell immunity, as well as explore their relevance as therapeutic targets in GVHD and other immune disorders. To accomplish this goal, we will investigate two areas of fibroblastic cell immunobiology in two independent Aims. Aim 1 will assess how myeloablative conditioning affects fibroblastic stromal cells as sources of Notch ligands during allo-BMT. We will study how graded intensities of myeloablative conditioning regulate Notch ligand expression and accessibility in subsets of fibroblastic stromal cells and other cell populations that interact with donor T cells. We will determine in real-time if myeloablative conditioning changes the behavior of alloantigen-specific T cells and their early interactions with host fibroblastic and hematopoietic cells. Using clinically relevant mouse allo- BMT models, we will test if fibroblastic cells remain key pathogenic sources of Notch signals driving GVHD even with reduced or no conditioning, which would suggest key functions in broader aspects of immunity. Aim 2 will test if fibroblastic stromal cells function as alloantigen-presenting cells during GVHD. Preliminary evidence suggests that fibroblastic stromal cells upregulate their antigen presentation machinery after allo- BMT. We will investigate the signals that drive this upregulation and use genetic loss of function approaches to test if fibroblastic cell-intrinsic MHC Class II expression contributes to GVHD pathogenesis. In parallel, we will use intravital imaging to study the impact of alloantigens and Notch ligands on early interactions between alloreactive T cells, fibroblastic stromal cells and other antigen-presenting cells at the onset of GVHD. My work has the potential to uncover new functions of fibroblastic stromal cells in GVHD and to suggest a role in broader immunological contexts. In addition, it will provide me with structured learning opportunities in experimental immunobiology that serve my training goals as an immunologist and physician-scientist.